The present invention is directed to amplifiers, and more particularly to a low distortion amplifier and method of reducing distortion in the amplifier during high output power events.
Some amplifiers spend substantially more time operating at relatively low output power than at a higher output power. Nevertheless, the goal of the amplifier designed is to provide low total harmonic distortion regardless of the output power. Typically this goal is achieved by overdesigning the amplifier to accommodate the highest anticipated output power. Such overdesigned amplifiers are able to provide low distortion for low and high signal levels at low and high output power. However, the overdesign is manifested by the excess dissipation of power when the amplifier is providing signals at low output power. If the need for high output power is rare (e.g., some video amplifiers are called upon to provide high output power for about 1 .mu.sec only one out of 10 million times), the waste of energy can be significant and is desirably avoided.
The present invention is directed to a low distortion amplifier and method of reducing distortion which avoids large power dissipation for low signal levels at low output power by dynamically adjusting the bias provided to an input stage of the amplifier based on a feedback from the output stage.
Accordingly, it is an object of the present invention to provide a novel method and amplifier which augments a quiescent (low output power) bias at high output power to thereby obviate the problems of the prior art.
It is a further object of the present invention to provide a novel method and amplifier which provides low distortion and low power dissipation at low signal levels, and low distortion and high power dissipation at high signal levels.
It is yet a further object of the present invention to provide a novel dynamically biased, low distortion, feedback amplifier in which total harmonic distortion is reduced by varying the bias currents of the analog amplifier.
It is another object of the present invention to provide a novel amplifier and method in which a current less than and proportional to an output current from the amplifier is sensed and provided to the input stage to augment the quiescent bias current at higher output power in order to operate the amplifier without substantial distortion at large input signal levels and high output power.
It is yet another object of the present invention to provide a novel amplifier with a bias circuit providing a quiescent bias current; an input stage with two pairs of input transistors, first and second level adjusting transistors each coupled to common bases of a different pair of input transistors, a first node coupled to a base of the first level adjusting transistor, and a second node coupled to a base of the second level adjusting transistor; a sensor for sensing a current less than and proportional to an output current from the amplifier; and first and second current mirrors each coupled to the sensor and to one of the first and second nodes, the first current mirror providing the sensed current to the input stage to augment the quiescent bias current at higher output power and the second current mirror thereafter removing the sensed current, thereby not affecting the quiescent bias which is used for low output power.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.